The terms: beads, microspheres or spheres are used here interchangeably to mean an approximately spherical object with a diameter from 1 micrometer to several millimeters.
Retro-reflective sheeting has been useful for various applications for decades. Typical applications include signage, such as traffic signs and marks, airport runway paints, etc., and non-signage applications, such as positioning markers, tagging-tracking-and-locating (TTL) applications. While the prismatic (corner cubes) retro-reflector provides high intensity reflection, the beaded or micro-sphere based retro-reflective sheeting remains a viable solution for many practical uses, due to its low cost, low maintenance, ease of application, and wider acceptance angle.
Beaded retro-reflective sheeting articles usually use a plurality of glass or other ceramic microspheres to reflect incidence light back in the direction from where it comes. Typically the microspheres are embedded in a substrate material, and a specular reflective coating is provided in between the microspheres and the substrate. The reflective coating can be a metallic layer, as described in U.S. Pat. No. 3,700,478 (Bingham), an inorganic dielectric coating, as described in U.S. Pat. No. 3,700,305 (Bingham), or an organic reflective coating having different refractive indices, as disclosed in U.S. Pat. No. 6,172,810 B1 (Fleming, et al).
The beaded retro-reflective sheeting can be categorized into three types: exposed lens, enclosed lens, and encapsulated lens. The exposed lens type provides the highest reflectance, but its brightness is easily reduced by foreign materials, especial by liquids such as water and dirt deposits on the exposed beads surfaces. The two enclosed types both have a protective material that is at least partially transparent to the wavelength of interest and they are above the front side of the beads: the embedded type provides good environmental protection compared to the exposed type, but has a reduced reflectance due to reduced index difference/planarization of the sheeting surface. The encapsulated type has a polymer layer covering the front side of the beads but leaves an air gap in between the cover layer and the beads. The latter configuration provides environmental protection without reducing the brightness.
All prior arts of beaded retro-reflective sheeting are polarization insensitive (incident or retro-reflected light are not polarized). For applications such as target recognition, a polarization sensitive retro-reflective article is highly desirable.